The present invention generally relates to nondestructive inspection methods and systems. More particularly, this invention relates to a method and system for scanning a turbine wheel with an eddy current probe, and particularly surface regions of slots within the wheel.
Various nondestructive examination (NDE) techniques have been used to perform nondestructive testing on articles. An example is eddy current probe inspection of turbine components, as disclosed in commonly-assigned U.S. Pat. Nos. 4,706,020, 6,426,622, and 6,545,467, whose disclosures pertaining to the construction, operation, and use of eddy current probes are incorporated herein by reference. A component of particular interest is industrial gas turbine wheels to which the buckets of the turbine are mounted. In the hostile operating environments of gas turbines, the structural integrity of the turbine wheels within its turbine section is of great importance in view of the high mechanical stresses that wheels must be able to continuously withstand at high temperatures. The regions of a wheel forming the slots into which the buckets are secured, typically in the form of what are known as dovetail slots, are known to eventually form cracks over time, necessitating monitoring of the wheel in these regions. In some wheel designs, such as the stages 1, 2, and 3 wheels of the General Electric 7FA gas turbine, cooling of the buckets and wheel perimeter is assisted by the presence of a cooling slot located near the perimeter of the wheel and into which the dovetail slots extend. Over extended periods of time under the severe operating conditions of a wheel, cracks may form at common edges formed where the dovetail slots and cooling slot intersect. The ability to detect cracks with lengths of as little as 60 mils (about 1.5 mm) and even less is desirable in order to provide sufficiently early detection to avoid catastrophic failure of turbine wheels.
While a turbine rotor can be completely disassembled to gain access to its individual wheels, inspection techniques that can be performed with limited disassembly are preferred to minimize downtime, such as to fit within outage schedules of a gas turbine employed in the power generating industry. Since buckets are typically removed for inspection, it would be preferred if the dovetail and cooling slots of a turbine wheel could be examined with only the buckets removed. However, access to the cooling slot is very limited, and any inspection technique using an eddy current probe must address the difficulty of bringing the probe into stable, near-proximity to the surfaces being tested.